Particle accelerator having beam deflecting means



April 9, 1968 P. J. GRATREAU 3,377,563

PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS Filed June 15, 1964 8 Sheets-Sheet 1 FIGJ 7 INWS TOK PIRR T- G nnrruinu QM a. jig

April 9 1968 P. J. GRATREAU 3,377,563

PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS Filed June 15, 1964 8 Sheets-Sheet 2 FIG.2

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April 1968 P. J. GRATREAU 3,377,563

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April 9, 1968 P. J. GRATREAU 3,377,563

PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS Filed June 15, 1964 8 Sheets-Sheet 5 IN uE/vrag PIERKE J". GRATRGAU i-rro Ngy April 9, 1968 P. J. GRATREAU 3,377,563

PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS 8 Sheets-Sheet 6 Filed June mdim INVENTOK PIERRE J. GRATREAU r 017-6 .ue/

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April 9, 1968 P. J. GRATREAU PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS 8 Sheets-Sheet 8 Filed June 15, 1964 United States Patent ()fifice 3,377,563 Patented Apr. 9, 1968 3,377,563 PARTICLE ACCELERATOR HAVING BEAM DEFLECTING MEANS Pierre J. Gratreau, Les Dauphins, Rue La Fontaine, Antony, France Filed June 15, 1964, Ser. No. 375,132 Claims priority, application France, June 14, 1963, 938,177 12 (Ziairns. ((31. 328237) ABSTRACT OF THE DISCLOSURE The invention provides means for increasing the beam current in a circular particle accelerator such as a beta tron having an annular vacuum chamber, an electron gun and a conventional beam guiding field, said means comprising a toroidal azimuth field winding wound around the vacuum chamber for focusing the beam and means for producing an electric field substantially perpendicular to the central plane of the chamber in the whole circumference thereof for deflecting the focused beam so that it misses the gun during a number of consecutive revolutions.

This invention relates to particle accelerators of circular form, more particularly to improvements in or relating thereto with a view to considerably increasing the strength of a particle beam injected into them.

The invention is especially useful in conjunciion with a betatron accelerator, which is considered a very good example because, among known accelerators, betatrons have the most intense beams; however the invention is not limited to use with a betatron since, as will appear from the description of the invention below, these improvements are capable of being used to increase the strength of the injected particle beam with most circular accelerators.

A betatron accelerator comprises an annular vacuum chamber disposed coaXially in a variable magnetic field which has a shape of revolution which is symmetrical with respect to the center plane of the chamber; the magnetic field of the betatron has two components for providing the field at the orbit and the central flux required for acceleration. The strength of the field is maximal through the central aperture of the annular chamber and decreases radially towards the chamber periphery so that electrons injected tangentially to an injection orbit, for instance, a peripheral orbit, of the chamber by an electron gun, can be accelerated at a constant radius on a privileged orbit known as the betatron orbit. The betatron orbit is kept stable by a barrel-lil e bending of the lines of force of the magnetic field so that the injected electrons tend to move towards an equilibrium orbit by amped oscillations; however, this focusing is slight. Since the apparatus is supplied by the oscillating discharge of a bank of capacitors, consecutive acceleration cycles can occur during each half-cycle of said oscillating discharge, and the effectiveness of injection is limited to a relatively short acceptance time at the start of each acceleration cycle, at a time when the field is within limits such that the radius of the instantaneous equilibrium orbit of the electrons leaving the gun lies between the injection orbit radius and the betatron orbit radius. Experiments have shown that injection efiiciency increases in proportion to the injection energy, and the electron beam current conventionally produced by betatrons is normally several tenths of an ampere. However, most of the electron beam delivered by the gun during the acceptance time is lost, more particularly on the chamber walls during the first revolution and on the back of the gun during the subsequent revolutions.

In conventional betatrons whose principles have just been outlined, the magnetic field is produced as a general rule by magnetic windings, and the magnetic field fulfills three functions. Its instantaneous value serves as a field for guiding the particles inside the chamber and as a focusing field for limiting the amplitude of particle oscillations around their instantaneous equilibrium orbit and thus preventing them from being lost by electrostatic repulsion. By its variation the magnetic field accelerates rotation of the particles and makes their instantaneous equilibrium orbit tend towards the betatron orbit. During the acceptance time the shift of the instantaneous equilibrium orbit and the variation of the magnetic field producing such shift can in a first approximation be assumed to be negligible as compared with the guiding effect of the same field, but the guiding effect is fairly small and the focusing elfect is not enough to prevent most of the electrons from the gun from being lost on the chamber walls during the first revolution of the beam. Also, the capture of orbiting electrons which remain after the first revolution can occur only to the extent that such electrons are not lost at the back of the gun during some subsequent turn or revolution.

In accordance with the present invention an accelerator for charged particles has a circular vacuum chamber in which particle acceleration takes place, a gun for marginal injection of particles to be accelerated into the chamber, means for producing a particle guiding field in the chamber, means for producing an azimuth focusing field in the chamber for confining injected particles near their instantaneous orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, means for generating a temporary field transversely of the azimuth field to deflect a focused particle beam along a path which misses the electron gun; the arrangement being such that reinforcement of the beam can be achieved by particle injection during successive revolutions of the beam' The present invention also includes a method of increasing the strength of a beam of charged particles in a particle accelerator having a particle-accelerating chamber of circular form, comprising producing a beam guiding field in the chamber into which the particles are marginally injected by a gun, producing an azimuth focusing field in the chamber confining the injected particle beam near its instantaneous orbit so that loss of particles caused by collision with the chamber walls is reduced, and temporarily producing a beam-deflecting field in the chamber transversely of the azimuth field to deflect the-focused beam so that it misses the gun during consecutive revolutions and thus enable injection to take place during consecutive revolutions of the particle beam to increase its strength.

An advantage of the invention is that it enables the beam current in circular particle accelerators to be increased. A further advantage is that the electron injection efficiency of betatrons can be improved.

In a first embodiment of a betatron according to the invention, the injected electron beam is guided by a field in known manner and is also focused by a substantially static field. The beam is deflected to miss the electron gun during the first revolution by a temporary electrostatic field produced by bringing two electrodes, disposed on-the wall of the annular vacuum chamber substantially on either side of the central plane thereof, to symmetrical potentials.

In a modified form of the first embodiment, the electron beam is injected tangentially to a peripheral orbit of the chamber and a third electrode is provided, adjacent the chamber wall near the axis of revolution thereof and adapted to be earthed for slowing down radial slip of electron beam as it approaches said third electrode.

In a second embodiment of a betatron according to the invention, the beam-deflecting field is induced by supplying the azimuth field winding at a rapidly varying rate so as to produce an increasing magnetic field; and the beam-guiding field is maintained at such a value during the injection step that the equilibrium orbit is near the betatron orbit. If the beam is injected at a tangent to a peripheral orbit of the chamber, the shift, which is iniitially perpendicular to the central plane of the chamber and which is due to the restoring force produced by the guiding field towards the equilibrium orbit, enables the beam to miss the electron gun during the first revolution, so that the induced electric field can be much less than the electrostatic field of the first embodiment.

In a modified form of the second embodiment, the

beam is injected substantially along a meridian plane of the annular chamber tangentially to a circle centred on the equilibrium orbit disposed at the centre of the chamber, at a rate corresponding to the cyclotron frequency of the azimuth field at a position between the electron gun and the equilibrium orbit. The invention leads to a strong circular particle beam, more particularly an electron beam, and can be used for all the uses of controlled-radiation sources, and more particularly of the betatron whose efficiency the means hereinbefore described improve. A beam of this kind can also be used either for studies of relativist plasma after neutralisation by ionisation of the molecules of residual gas in the chamber or, if the particles have sufiiciently low energy, to enable the betatron to operate as a plasma betatron.

The invention will now be described in more detail, by way of examples, with reference to the accompanying mainly diagrammatic drawings, in which:

FIG. 1 is a diagrammatic plan view of a betatron;

FIG. 2 is a section taken along the line 11-11 of FIG- URE 1;

FIG. 3 is a partially block diagram of the supply cir cuits for the betatron;

FIG. 4 shows curves illustrating the injection process;

FIG. 5 is an explanatory diagram of a capture process in a betatron;

FIG. 6 is an explanatory diagram similar to FIG. 5 showing a different captive process with a modified betatron;

FIG. 7 is a view corresponding to FIG. 2 of an alternative arrangement of betatron;

FIG. 8 is a partially block diagram for the supply circuits for the betatron shown in FIG. 7; and,

FIG. 9 is a diagram illustrating by curves the injection process in the betatron of the kind shown in FIG. 7.

The betatron shown in FIGS. 1 and 2 is a large-chamber air betatron, of the kind described in an article written by the present applicant and by Charles Maisonnier and published under the reference 60-1 by the Nuclear Study and Research Centre (CERN) at Geneva in 1960, the improvements according to this invention having been added to this known betatron. The published characteristics of this betatron relate mainly to the dimensions of its toroidal vacuum chamber 1, which hasa relative opening of approximately 0.4, and to its guiding and accelerating winding 2, 2' whose turns 2a-2l, 2a, 2'l are connected in series and disposed on a toroidal surface covering the vacuum chamber symmetrically in relation to its central plane so that the winding has optimum coupling with an equilibrium orbit 3 and corresponds to a very wide po-' tential sink. In order not to overload the drawing of FIG. 1, only the turns 2a, 2 21 of this winding are shown by way of example with its input connection 21 and output connection 22. An electron gun 4 is arranged to inject electrons tangentially to a peripheral orbit of the chamber in which a vacuum is maintained through a conduit 5.

The novel features of this betatron are the addition of an azimuth field winding 6 which. extends around the vacuum chamber and has a circular axis concentric of the same, and the addition of two electrodes 7 ,7 which are disposed on the chamber walls so as to produce when symmetrical potentials are applied to them, an electric field substantially perpendicular to the central plane of the chamber.

The winding 6 is formed by turns which can be substantially square and which are connected in series, for instance, by junctions between one end of their central branch parallel with the axis of revolution of the chamber and the corresponding radial branch of the nextturn. All these junctions together form a loop which may produce a disturbing field in the betatron, and so a compensating loop (not shown) can be provided nearby. Winding 6 is supplied through conductors 61, 62 connected to the ends of a break in any of its turns.

The electrodes 7, 7 are conductive enough to be considered as equipotential and resistive enough for stray currents to be negligible and are in the form, for instance, of a very thin metal coating or-and preferably-wt a carbon compound coating such as the product designated by the trademark Aquadag. The electrodes 7, 7' have connections 71, 72 respectively to two terminals of a source for applying symmetrical potentials to them. For reasons which will be disclosed hereinafter, the electrodes 7,. 7 are not symmetrical with respect to the central plane of the vacuum chamber but are symmetrical with respect to a conical surface forming a desired angle, for instance, of approximately 40, with such plane.

FIG. 3 is a partially block schematic diagram showing the control circuitry for the betatron shown in FIGS. 1 and 2. An alternating current source is providedto heat the filament of the gun or particle source 4, through the agency of a transformer 401 and choke coils 402, and to apply symmetrical potentials to the electrodes 7, 7' via a grounded-center-point voltage multiplier 700 and a normally closed (no) contact of a triggering device symbolically shown as a selector switch 101 whose operative contact is connected in parallel with the control circuits during injection and, when the winding 2 is being used for acceleration, to the control circuit for the azimuth field winding 6, to the control circuit of the gun 4 and to the circuit for discharging the electrostatic-field electrodes 7, 7' via delay lines 201, 211, 601, 411, 701 which ensure that the variousitems come into operation in the order and at the times shown in FIG. 4. The delay lines respectively control spark gaps 203, 213, 603, 413, 703 via trigger circuits 202, 212, 602, 412, 702 which can be, for instance, thyratrons. The spark gap 603 is connected in series between the azimuth winding 6 and a capacitor 600 charged to a high voltage. The spark gap 203 is connected in series with a choke coil 214 between the betatron winding 2 and a capacitor 200 charged to a high voltage. The spark gap 213 is disposed between the winding 2 and a bank of capacitors 210 charged to a very high voltage. The choke coil 214 and a spark gap 215 protect the spark gap 203 and capacitor 200 against the discharge current of the main capacitor bank 210. The spark gap 413 is connected in series between the cathode of the gun 4 and a delay line 400 which can be,

for instance, a coaxial line charged to a high voltage and terminated at its characteristic impedance. The spark gap 703 is a double spark gap which is connected across the electrodes 7, 7' and which has its center point grounded.

As the graphical illustrations in FIG. 4 show in (a), the betatron winding 2 is supplied, via the capacitor 200 and the spark gap 203 triggered at a time t with an oscillating discharge at a frequency low. enough for its current to be taken as substantially constant and equal to its peak current during the time taken for injection; immediately after the completion of injection the winding 2 is supplied by the capacitor 210 via the spark gap 213 triggered at a time t.; at a rapidly increasing rate to accelerate the injected particles. Since this acceleration.

process is known and has nothing to do with the subject matter of this invention, only the initial acceleration conditions will be considered herein.

The azimuth field winding 6 is supplied in the manner shown by the graph b in FIG. 4 under critically damped conditions so that its current can be taken as constant and equal to the peak current during injection and varies little during the time taken by the field produced in the vacuum chamber by the acceleration current to become at least equal to the maximum azimuth field. The spark gap 603 is triggered at a time t arbitrarily taken as the time origin in FIG. 4 and so determined that the current discharged by the capacitor 200 through the winding 2 and the current discharged by the condenser 600 through the winding 6 are both at a maximum near the time at which injection is effected.

To this end, and as the graph 6 in FIG. 4 shows, a substantially rectangular pulse whose voltage and duration are determined by the charge and the length of the line 400 is applied to the gun 4 at a time t The voltage of this pulse is so related to the value of the guiding field at the time t that the equilibrium orbit is near the betatron orbit. At the time r when this pulse finishes, the spark gap 703 is triggered and short circuits the electrodes 7, 7, so that the electrostatic field produced thereby ceases, as the graph d in FIG. 4 shows. The injected beam is then captured in the chamber, as will be seen with reference to FIG. 5. Simultaneously or immediately afterwards-at the time t the main bank 210 discharges through the winding 2. and its voltage is high enough for the strength of the guiding field to have exceeded the strength of the azimuth field before the same has really started to decrease, so that further decrease of the azimuth field does not risk any loss of the beam whose various turns are concentrated near the equilibrium orbit during acceleration in known manner. The beam can then be deflected by an extra field in some conventional variation process and diverted against a target for use.

FIG. 5, which is a sectioned View of the vacuum chamber ll through the meridian plane containing the injector 4, shows the process for capturing electrons injected during several revolutions by the process hereinbefcre described. The guiding field produced by the betatron winding 2 derives from a vector potential which has a minimum on the betatron orbit, and so the guiding field forms in the meridian plane closed equipotential lines, as 24, around the trace 3 of the betatron orbit. The minimum azimuth field value produced by the winding 6 is so devised that it has sufficient confining effect on the beam to substantially prevent losses on the chamber walls during the first revolution of the beam. Superimposing the electrostatic field which is substantially perpendicular to the central plane of the chamber, as shown by field lines 73, and which derives from the potential difference between the electrodes 7, 7 makes the orbit of the electrons delivered by the gun 4 slip along a curve 74 slightly different from an electrostatic equipotential line, at a rate which depends upon the relationship between the electric field and the azimuth field and which is so chosen that the beam misses the gun during its first turn. Consequently, after an injection lasting, for instance, for 5 turns of the beam, the traces thereof on the meridian plane of the chamber are at A, B, C, D, and E respectively. The electric field is then suppressed in order that the beam which was injected during the first turn and whose trace is at A may not be lost on the chamber inner wall. Since the particles experience only the restoring force which is directed towards the equilibrium orbit and which gives their instantaneous orbits a slight slip along the magnetic equipotential lines 24, the particles continue to rotate in the chamber so that they are captured and can, for instance, be accelerated by a variation of the guiding field which also helps to concentrate them near the betatron orbit in a very strong beam. The azimuth field required to confine the injected particles initially is low enough not to upset this concentration of the various turns of the beam and its value becomes negligible relatively to the value of the guiding field at the end of an acceleration with sufficient energy. The difference between the curve 74 and an electrostatic equipotential line is due to the restoring force which is directed towards the equilibrium orbit and which initially tends to give the beam a slip taking it away from the central plane of the chamber. For optimum utilization of the chamber volume, the electrostatic field can be inclined to the central plane of the chamber by the electrodes 7, 7' being disposed obliquely to such plane.

For even better utilization of the chamber volume in a manner leading to injection of a greater number of beam turns, the electrostatic field electrodes can be arranged in the manner shown in FIG. 6. In this arrangement, the gun 4 injects along a peripheral orbit of the chamber and an electrostatic field concentrated in the peripheral zone thereof is produced by means of electrodes 70, 70', of symmetrical potentials, limited to this zone, and by one neutral electrode 70" which is disposed on the wall in the central zone of the chamber, preferably on the inside surface of such wall. The electrostatic field lines 75 are therefore deflected inwardly towards the neutral electrode 70" and the electron orbit slips along a line 76 at a rate which decreases simultaneously with the electrostatic field and which passes through a minimum near the betatron accelerating orbit 3. A greater number of beam turns than in FIG. 5 can therefore be injected before the first such turn has any chance of being lost on the chamber walls. When the electrostatic field is cut off, the various turns on the injected beam slip along magnetic equipotential lines 24' which become denser around the betatron orbit in proportion as they are nearer the same.

FIG. 7 is a diagrammatic view showing, in section through a meridian plane, a betatron according to a second embodiment of the invention wherein the beam-deflecting electric field is produced by a rapid variation of the azimuth field. In order that the orbit for zero induced electric field may be at the centre of a vacuum chamber 10, each turn of an azimuth field winding 66 has substantially the shape of a semi-ellipse 61 whose minor 'axis coincides with the axis of revolution of the chamber 10, and the same has a mean radius of some 75% of half the major axis of the ellipse. The meridian trace of the central orbit of the chamber is therefore near the centre of curvature of the ellipse at the top of its major axis, and the lines of force of the electric field induced near this zero-induced-field orbit are substantially circles 31 centred around the trace 30. The inside of the chamber 10 is protected against the electrostatic field produced inside the azimuth field winding 60 when a current flows therethrough by an electrostatic screen 11 W'h0S6 resistivity is constant and so calculated that stray currents flowing through it are small enough not to superimpose any appreciable electrostatic field upon the induced electric field. The screen 11 is fitted to the wall of the vacuum chamber 11) which in shape substantially resembles an annulus centred on the zeroinduced-field orbit 30. To reduce the inductance of the azimuth field winding all the turns thereof are supplied in parallel by one central conductor 61. A winding of this kind, which for the sake of convenience can be of polygonal form as shown in FIG. 7, is preferably associated with a guiding-field winding whose turns Ila-21. Z'a-Zl are arranged similarly to what has been described with reference to FIGS. 1 and 2.

FIG. 8 is a circuit diagram showing the control circuits for betatron in FIG. 7. The only difference from FIG. 3 is the omission of the electrodes 7, 7 found therein and of their supply and discharge circuits, and the addition of a spark gap 613 for short-circuiting the azimuth field winding 60, the spark gap 613 being controlled via a delay circuit 611, by means of a trigger circuit 612.

7 Like elements in FIGS. 3 and 8 therefore have the same references and will not be re-described.

FIG. 9 shows the time relationships, in one operating cycle of the betatron shown in FIG. 7, between the roduction of the guiding field, the acceleration by the betatron winding 2 (graph a), the production of the azimuth field by the winding '60 (graph b) and the injection pulse applied to the gun 4 (graph Closure of the switch 102 produces:

With effect from a time t' arbitrarily taken as the time origin, an oscillating discharge of the capacitor 200 through the winding 2 in conditions similar to those described with reference to FIG. 4;

An oscillating discharge at a relatively high frequency of the capacitor 600 through the winding 60 at a time t; such that the azimuth field is rapidly increasing while the guiding field is steady approximately at its peak value;

The application to the gun 4 of a rectangular pulse for discharging the coaxial line 400 from the time 1' when the azimuth field is strong enough to effectively focus the beam from the initial time, this azimuthal field strength being approximately twice the peak value of the guiding field; this pulse ends at the time 1' before the end of the growth of the azimuth field;

Immediately after the termination of injection, at a time t',;, the discharge of the main accelerating bank 210 through the betatron winding 2, and,

At the time it' when the azimuth field reaches its peak value, the triggering of the spark gap 613 which shortcircuits the winding 69 to slow down the decrease of the azimuth field so that the same passes through a value nears its value at the end of injection only after sufficient time has passed for the guiding field to have exceeded such value and to have accelerated the beam to several Mev.

Since the azimuth field is several times as strong as the guiding field during the injection pen'od, it can be deduced from the expression of the centripetal slip rate of the beam in dependence upon the ratio between such fields that the movement of the beam towards the equilibrium orbit proceeds substantially with a constant azimuth flux.

When the beam is injected tangentially to a peripheral orbit of the chamber as in the first embodiment, the initial vertical slip due to the restoring force acting towards the equilibrium orbit is enough for the beam to miss the gun on its first revolution, so that in continuous operation the beam would pass by the gun only after a number of revolutions, so that the induced electric field can be much less than the electrostatic field of the first embodiment. For improved beam focusing the gun can be magnetically screened by a sufficiently thick conductor.

In a modified form of operation, the beam can be injected in a direction having a small component tangential to a peripheral orbit of the vacuum chamber and a preponderant meridian component in which the injection speed corresponds to the cyclotron frequency of the azimuthal field. As with tangential injection, the beam progresses towards the inside of the chamber with a substantially constant flux.

The non-uniformity of the azimuth field causes axial slip of the beam, and this can be compensated for by an electrostatic field being applied radially and varying in proportion to the azimuth field. Accordingly, the electrostatic screen 11 is divided to form two opposing electrodes.

The injection conditions hereinbefore described can be produced by moderate potentials and fields, the intensity of the particle beam being mainly due to the fact that injection etficiency is substantially 100%. In particular, the particles do not have to be injected with so much energy as in conventional betatrons, so that the electron gun voltage can be modulated to act on the position of the equilibrium orbit during the injection period.

To give some idea of values, some actual field strength 8 a and voltages are shown which have been used to capture the injected electron during five beam revolutions in a betatron as shown in FIGS. 1 and 2 having a betatron orbit of approximately 1 metre in circumference. In these experimental conditions a measured beam strength of greater than 1 ampere has been found but of course the invention makes it possible to inject a far greater number,

of beam turns and therefore to produce a very much greater beam current.

What I claim is:

1. An accelerator for charged particles, having an annular vacuum chamber in which particle acceleration takes place, a gun for injecting particles to be accelerated into the chamber, means for producing a particle guiding field in the chamber, means for producing an azimuth focusing field in the chamber for confining injected particles near their instantaneous orbit so that loss of particles caused by collision with the chamber walls is substantially avoided, and means for generating a temporary field extending over the circumference of said chamber transversely of the azimuth focusing field to deflect the focused particle beam along a path which misses said gun Whereby reinforcement of the beam can be achieved by particle injection during successive revolutions of the beam.

2. An accelerator for charged particles, having an annular vacuum chamber in which particle acceleration takes place, a gun for injecting particles to be accelerated into the chamber, means for producing a particle guiding field in the chamber, means for producing an azimuth focusing field in the chamber for confining injected particles near their instantaneous orbits so that loss of particlcs caused by collision with the chamber walls is; substantially avoided, two electrodes with extended surface areas adjacent the chamber wall over the circumference of said chamber, and means for bringing said electrodes to symmetrical potentials generating a temporary field transversely of the azimuth field to deflect the focused particle beam along a path which misses said gun whereby reinforcement of the beam can be achieved by particle injection during successive revolutions of the beam.

3. An accelerator for charged particles, havingan annular vacuum chamber in which particle acceleration takes place, a gun for injecting particles tangentially to a peripheral orbit of the chambenmeans for producing a particle guiding field in the chamber, means for producing an azimuth focusing field in the chamber to confine injected particles near their instantaneous orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, first and second electrodes with extended surface areas adjacent the chamber Wall over the circumference of said, chamber, said first and second electrodes being provided with energizing means whereby said electrodes are adapted to be broughtto symmetrical potentials for generating a temporary field transversely of the azimuth field to deflect a focused particle beam along a path which misses said gun, and a third electrode adjacent the chamber wall near the axis of revolution thereof and adapted to be held at a potential between the potentials of said first and second electrodes whereby said temporary beam-deflecting field is maximal near the peripheral injection orbit and gets progressively weaker toward said third electrode thereby slowing down radial slip of the particle beam as it approaches said third electrode.

4. An accelerator for charged particles having an annular vacuum chamber in which particle acceleration takes place, a gun for injecting particles tangentially to a peripheral orbit of the chamber, means for producing a particle guiding field in the chamber, means for producing an azimuth focusing field in the chamber to confine injected particles near their instantaneous orbit thereby avoiding loss of particles by collision withthe chamber walls, first and second electrodes with cxtended surface areas adjacent the chamber walls over the circumference of said chamber, and symmetrically disposed on opposite sides of a plane inclined to the central plane of the vacuum chamber, and a third electrode adjacent the chamber wall near the axis of revolution thereof, said first and second electrodes being energized and adapted to be brought to symmetrical potentials for generating a temporary field transversely of the azimuth field to deflect a focused particle beam along a path which misses said gun, and said third electrode being adapted to be held at a potential between the potentials of said first and second electrodes whereby said temporary beam-deflecting field is maximal near the peripheral injection orbit and gets progressivei y weaker toward said third electrode thereby slowing down radial slip of the particle beam as it approaches said third electrode.

5. An accelerator for charged particles, having an annular vacuum chamber in which particle acceleration takes place on an accelerating orbit, a gun for injecting particles to be accelerated into the chamber, means for producing a particle guiding field in the chamber, a field winding adapted to be excited at a rapidly increasing rate for producing on the one hand an azimuth focusing field in the chamber which confines the injected particles near an equilibrium orbit thereby avoiding loss of particles by collision with the chamber walls, on the other hand a temporary field extending tranversely of the azimuth field over the circumference of said chamber to deflect a focused particle beam along a path which misses said gun, and said means for producing a particle guiding field maintaining said guiding field at such a value during particle injection that said equilibrium orbit is near said accelerating orbit.

6. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particles are accelerated along a predetermined constant radius equi ibrium orbit, injector means for injecting particles into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said particles, means for producing a particle guide magnetic field in the chamber, means for producing an azimuth focusing magnetic field inside said chamber having a toroidal shape with respect to the revolution axis of the chamber for confining during said time the injected particles along their injection orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, and means for generating during said time a temporary electric field transverse to the equatorial plane of the chamber over the whole circumference thereof to deflect the focused particle beam along successive orbits which all miss said injector means.

7. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particles are accelerated along a predetermined constant radius equilibrium orbit, injector means for injecting particles into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said particles, means for producing a particle guide magnetic field in the chamber, means for producing an azimuth focusing magnetic field inside said chamber having a toroidal shape with respect to the revolution axis of the chamber for confining during said time the injected particles along their injection orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, two electrodes against the walls of said chamber substantially symmetrical with respect to the equatorial plane of the chamber over the whole circumference thereof and means for generating during said time a temporary electric field between said electrodes to defiect the focused particle beam along successive orbits which all miss said injector means.

8. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particles are accelerated aiong a predetermined constant radium equilibrium orbit, injector means for injector particles into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said particles, means for producing a particle guide magnetic field in the chamber, a toroidal azimuth magnetic field winding wound around the chamber, means for energizing said winding during said time for confining the injected particles along their injection orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, two electrodes against the wa.ls of said chamber substantially symmetrical with respect to the equatorial plane of the chamber over the whole circumference thereof and means for generating during said time a temporary electric field between said electrodes to deflect the focused particle beam along successive orbits which all miss said injector means.

9. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particles are accelerated along a predetermined constant radius equilibrium orbit, injector means for injecting particies into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said paricles, means for producing a particle guide magnetic field in the chamber, means for producing an azimuth focusing magnetic field inside said chamber having a toroidal shape with respect to the revolution axis of the chamber for confining during said time the injected particles along their injection orbits so that loss of particles caused by collision with the chamber walls is substantially avoided, three electrodes against the wall of said chamber over the whole circumference thereof, the two first of said electrodes being located against the outer periphery of the annular chamber and symmetrical of each other with respect to the equatorial plane of the chamber and the third electrode being located against the inner periphery of the annular chamber symmetrically with respect to said equatorial plane and being brought to a potential equal to the mean value of the potentials of the two other electrodes, and means for generating during said time a temporary electric field between said two first electrodes to deflect the focused particle beam along successive orbits which all miss said injector means.

10. A circular accelerator for accelerating charged par- 'cres comprising an annular vacuum chamber in which particles are accelerated along a predetermined constant radius equilibrium orbit, injector means for injecting partic'es into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said particles, means for producing a particle guide magnetic field, means for producing an azimuth focusing magnetic field inside said chamber having a toroidal shape with respect to the revolution axis of the chamber for confining during said time the injected particles along their injection orbits so that loss of particles caused by collision with the chamber walls is substantially avoided and means for varying at a rapidly increasing rate said azimuth focusing magnetic field to derive therefrom a temporary electric field having substantially circular lines of force centered on the equilibrium orbit to deflect the focused particle beam along successive orbits which all miss said injector means.

11. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particies are accelerated along a predetermined constant radius equilibrium orbit, injector means for injecting particles into the chamber tangentially to a peripheral orbit during a time substantially greater than one orbital period of said particles, means for producing a particle guide magnetic field in the chamber, an azimuth magnetic field winding wound around the chamber, the turns of said winding being located in radial planes passing through the revolution axis of the chamber and having in said planes the shape of halves of flattened ellipses the small axis of which coincide with said revolution axis, and means for varying at a rapidly increasing rate the current in said winding for first producing a variable magnetic field for confining the injected particles along their injection orbits so that loss of particles caused by collision 11 with the chamber walls is substantially avoided and secondly for deriving from said rapidly variable magnetic field a temporary electric field having substantially circular lines of force centered on the equilibrium orbit to deflect the focused particle beam along successive orbits which all miss said injector means.

12. A circular accelerator for accelerating charged particles comprising an annular vacuum chamber in which particles are accelerated along a predetermined constant radius equilibrium orbit, a gun for injecting particles to be accelerated into the chamber during a time substantially greater than one orbital period of said particles, in a direction providing a small component tangential to a peripheral orbit and a large meridian component, a toroidal field winding wound around the chamber, means for energizing said toroidal field winding at a rapidly increasing rate for producing an azimuth focusing field 12 in the chamber which confines the injected particles near an equilibrium orbit and :a temporary field extending transversely of the azimuth field over the whole circum ference of the chamber to deflect a focused particle beam along successive orbits which all miss said gun, the injection speed of said gun corresponding to the cyclotron frequency of said azimuth field at a position located between the gun and the equilibrium orbit,

References Cited UNITED STATES PATENTS 2/1952 Wideroe 313-62 X 9/1959 Bennett 31362 

